XXE in The Opennms Group Horizon

CVE-2026-19596

An XML External Entity (XXE) vulnerability exists in the XML collector of multiple versions of OpenNMS Meridian and Horizon. When OpenNMS collects XML from a source whose response is attacker-controlled (for example a compromised monitored host or an HTTP man-in-the-middle position), the collector's XML parser resolves external entities and external DTDs. This allows an attacker to read files accessible to the OpenNMS service account, including database credentials, and to induce out-of-band requests. The solution is to upgrade to Meridian 2024.3.13, 2025.0.10 and Horizon 36.0.4 or newer. Meridian and Horizon installation instructions state that they are intended for installation within an organization's private networks and should not be directly accessible from the Internet.

Vulnerability class: XXE (XML External Entity)

CVSS v3 metric

CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N.

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-19596?
CVE-2026-19596 is a medium-severity vulnerability in The Opennms Group Horizon, classified under Improper Restriction of XML External Entity Reference (XXE). CVSS score: 5.9/10. Published 2026-09-10.
How severe is CVE-2026-19596?
Medium severity. CVSS v3 base score is 5.9 out of 10.